1. Field of the Invention
This invention relates to mesomorphic polyisocyanate compositions and their use in a liquid-crystal containing structure in a device and process for displaying indicia, performing memory functions or indicating temperature.
2. Brief Description of the Prior Art
Mesomorphic materials, also known as liquid-crystalline materials, are organic compounds which can exist in a transition state, i.e. a mesomorphic state, of physical structure between normal crystalline solid and isotropic liquid forms.
It is known in the prior art as described in U.S. Pat. No. 3,935,337 (1976) to use liquid-crystalline materials in electronic display or memory devices. Such devices typically comprise a thin layer of liquid-crystals sandwiched between two sheets of glass. Normally, the thin layer of liquid-crystal material is clear, but under the application of an electric field or heat, some regions of the material become turbulent so as to scatter light. By controlling the size and shape of the turbulent regions, images can be formed as a result of the light scattering. Primarily, this effect is obtained by use of liquid crystal materials of the nematic type. By the term "nematic" is meant a particular mesomorphic state, which is the first state formed upon cooling from a liquid melt and in which the orientation of the molecules is in parallel line but not layers.
Materials exhibiting liquid-crystalline behavior are well known in the prior art, and are generally divisible into three categories: (1) relatively stiff elongated molecules, non-polymeric in nature, such as 4,4'-azoxyanisole (2) polymeric liquid crystals whose pendant groups (side chains) are mesogenic in nature and whose backbone is flexible in nature, such as poly[di(N-p-acryloyloxybenzylidene)-p-diaminobenzene] and (3) aromatic, rigid and pendantless polymers such as polyamides or polyesters, such as poly(1,4-phenylene terephthalamide) as a concentrated solution in sulfuric acid. By the term "mesogenic", as used herein, is meant materials which can exist in mesomorphic modification.
It is generally accepted that the combination of geometrical shape and molecular polarity are the parameters most responsible for inducing liquid-crystalline behavior.
Polymers, with liquid crystal side groups, as described above, are either polyacrylic (polyacrylates, polymethacrylates and their copolymers) or polyvinyl (polystyrene) in nature. See S. B. Clough, A. Blumstein and E. C. Hsu, Macromolecules 9, 123 (1976); V. N. Tsvetkov et al., Europ. Polymer J. 9, 481 (1973); L. Strzelecky and L. Liebert, Bull. Soc. Chim. France (1973), p. 597. In all of the above polymers, the backbones are flexible and do not impart directionality to the pendant residues. Pendant groups on the polymers are usually chosen from among monomeric species that by themselves exhibit liquid-crystalline behavior, or are mesogenic in nature. Upon polymerization, these pendant groups tend to align themselves in an ordered fashion to cooperatively act as liquid crystals. The spatial positioning of such cooperative regions is not controlled by the polymeric backbone, due to the flexibility of the latter, except for having the pendant groups at fixed distances from the meandering, flexible backbone. Directionality and cooperative phenomena, if they exist in such systems, are due solely to the inherent properties of the monomeric species and are independent from the random path of the flexible backbone chain.
In the continuing search for materials exhibiting liquid crystalline behavior, what is desired in the art is a class of materials in which the variables affecting desirable liquid-crystal behavior can be conveniently and predictably monitored and "tailored" for respective end uses. Particularly in a polymeric system, what is desired is a rigid and extended polymeric backbone (albeit not necessarily as a rigid rod), in which the positions of selected pendant groups can be spatially fixed relative to one another, along the same chain. The pendant groups should be flexible or partly flexible in nature, and exhibit neither mesomorphic nor mesogenic behavior in the monomeric state, such that the primary parameter controlling liquid-crystalline behavior is the structural combination of the extended rigid polymer backbone with attached pendant groups.
Two examples of polyisocyanides, exhibiting liquid-crystalline behavior are poly(alpha-phenethyl isocyanide) and poly(beta-phenethyl isocyanide) as described in Adv. Polymer Sci., 19, 117 (1975). None of the other members of this class of polymers is reported to exhibit mesomorphic behavior. However, in view of the toxicity and health hazards associated with the above materials, their utility in commercially acceptable devices is probably severely limited.
Polyisocyanates are a known class of polymers and methods of preparation, physical and chemical properties thereof are described for example, in the references; Russian Chemical Reviews, Vol. 41(a) pp. 774-785 (1972); Macromol. Sci.-Revs. Macromol. Chem. C9 (2) pp. 269-303 (1973); and J. Am. Chem. Soc. 82, pp. 866-873 (1960). The reference, Macromolecules, Vol. 11, pp. 300-306 (1978) describes solution properties of poly(n-butyl isocyanate). The synthesis and properties of aromatic and extended chain polyamides is described in Macromolecules, Vol. 10, pp. 1381-1390 (1977). However, in none of the above references, are liquid-crystalline properties of polyisocyanates described.